An automobile air conditioning system is comprised of a compressor, a condenser, a restrictor, an evaporator, and an accumulator connected in an operative fashion or circuit. In prior art systems the evaporator is connected to the accumulator via a fluid line. The accumulator is usually mounted in the engine compartment of the vehicle. The function of the accumulator is to store or release refrigerant as required by system operating conditions. Also the accumulator stores an extra supply of refrigerant to make up for future system leakage. Another function of the accumulator is to separate liquid from gaseous refrigerant emanating from the evaporator. A non removable drier is usually incorporated in the accumulator. Prior art accumulators generally utilize a U-tube design in the outlet conduit within the shell which leads to the compressor suction line. This U-tube has a small opening near its lowest point which allows liquid containing oil to be drawn into the refrigerant flowing to the compressor. This "oil bleed hole" as it is commonly called is necessary for oil recovery. Without an "oil bleed hole" most of the system oil would end up in the accumulator thereby resulting in lack of lubrication to the compressor and causing its subsequent failure.
Most compressors in automotive use today require a continuous flow of oil for adequate lubrication since these compressors have no oil sump. Several decades ago compressors utilized the "oil sump design" but cost, size and extra weight caused its elimination. The oil flow to the compressor is not only from this "oil bleed hole" but also in the oil entrained in the suction gas.
The liquid refrigerant circulated from the oil bleed hole is detrimental to system performance as additional refrigerant must be compressed and condensed which has accomplished no cooling effect in the evaporator. This extra liquid flow results in more power to drive the compressor and reduced compressor capacity especially at idle and low speed hot ambient conditions where the compressor lacks capacity for maximum cooling as allowed by the evaporator coil antifreeze control point. At higher speeds the head pressure is raised by the excess flow but cooling performance does not usually suffer since the compressor capacity is greater than evaporator capacity. This is as compared to a thermostatic expansion valve system which only allows the exact flow to totally vaporize the refrigerant returning to the compressor.
The accumulator provided with a U-tube operates in an acceptable fashion in terms of oil recovery when the accumulator has liquid refrigerant within as it is designed to have. During system operation an oily froth exists due to turbulence of flow and boiling of refrigerant. This aids greatly in entraining oil in the suction gas. However, under low charge conditions where the gas leaving the evaporator has no liquid except for oil this froth tends to be greatly diminished. Now, the accumulator must supply oil primarily via the oil bleed hole. At this condition oil recovery suffers since the oil by itself as compared to being mixed with liquid refrigerant has less tendency to flow into the opening due to lower pressure differential across the opening and higher viscosity and entrained gas. Also a substantial quantity of oil must be deposited in the accumulator to reach the oil bleed hole, which can be located away from the bottom of the accumulator since a screen apparatus which is used requires some clearance.
It would be advantageous to incorporate an integral evaporator-accumulator in one package for purposes of achieving less cost, space, and for ease of installation at the final vehicle assembly plant. It would also be advantageous to eliminate the oil bleed hole and its aforementioned disadvantages and to improve low charge oil recovery. Also, a serviceable filter drier would be a benefit over prior art driers which cannot be replaced without replacing the whole expensive accumulator. Thermal losses would also be reduced by removing the accumulator from the hot engine compartment.
In U.S. Pat. No. 4,794,765, issued Jan. 3, 1989, to Thomas J. Carella et al., the U-tube oil bleed concept is used and the total package is approximately 20% (3") wider than the evaporator by itself. Since space is at a premium in car underpanel locations the extra width is a disadvantage as is the requirement for a separately installed liquid line filter drier which probably negated the cost savings of eliminating the installation labor of a separate accumulator.
Efforts to design a practical integral evaporator-accumulator have been continuing since the advent in automobiles in 1973 of the fixed restrictor refrigerant flow systems by General Motors Corporation. No American built vehicle has as yet commercially incorporated the concept of an integral evaporator-accumulator probably due to size and cost constraints.
U.S. Pat. No. 4,794,765 also discloses refrigerant recirculation from the accumulator to the evaporator. However, the U-tube and oil bleed were still used. One of the features of U.S. Pat. No. 4,794,765 was that "free" cooling would be one result which is grossly exaggerated as the compressor-condenser must create the liquid which is recirculated. Recirculation alone could result in more liquid being returned to the compressor and a subsequent loss in cooling performance. This is due to the fact that the oil bleed would still return the same amount as without recirculation and now the extra liquid emanating from the evaporator to the accumulator must be separated to a greater degree. This does not appear to be the case in U.S. Pat. No. 4,794,765.
Current evaporators used in automobiles generally do not suffer from poor refrigerant distribution due to use of a multiple pass design and coil flooding caused by the liquid return quantity to the compressor. Thus U.S. Pat. No. 4,794,765 offers few advantages and is more costly than the fixed restriction systems which are currently used.